The present invention relates to signal synchronization, and more particularly to audio frame synchronization for embedded audio demultiplexers that detects the boundary between audio frames to synchronize the embedded audio with the video.
Multiple channels of digital audio may be multiplexed with a digital video signal during the inactive period between the end of active video for one line (EAV) and the start of active video for the next line (SAV) for component video, or during the horizontal and vertical intervals for composite video, so that the combined signal is transmitted over a single video cable. At the receive end of the video cable the video and individual audio signals are separated by a demultiplexer. A proposed Society of Motion Picture and Television Engineers (SMPTE) standard 259M, "Formatting AES/EBU Audio and AES/EBU Auxiliary Data into Digital Video Ancillary Data Space", describes the number of digital audio samples from each audio channel per video frame in order to maintain a desired audio sample rate at the receive end of the video cable. When the audio sample rate is not an integer multiple of the video frame rate, several video frames may elapse before an integer number of audio samples coincides with a video frame boundary. The number of video frames required to match an integer number of audio samples at a given sample rate is called an audio frame.
Demultiplexers currently have no way of determining the phase of the audio frame. Consequently the demultiplexers randomly select a phase before beginning to demultiplex the embedded audio signal. Different demultiplexers select different audio frame phases, resulting in differing audio phases at their outputs. The phase shift due to audio frame misalignment may exceed two audio samples between separate demultiplexers. For example with an audio sample rate of 48 kHz the time difference between two misaligned audio channels is 20.83 microseconds per sample of difference. This time difference is quite small and not likely to be heard directly when the error is due to unsynchronized audio frames with an error of one to two samples. However the problem becomes apparent when two or more misaligned channels are mixed together. Therefore if two channels are mixed that contain some audio that is common to both channels and some audio that is different in each channel, which would happen with a stereo microphone setup, the phase difference due to the audio frame misalignment causes distortion in those audio components that are common to both channels. The problem is more acute at higher frequencies where the misalignment represents a higher percentage of the frequency's period. A two sample shift between the channels causes frequencies near 12 kHz to cancel almost entirely. Thus the effects of mixing misaligned channels may range from small phase changes to complete cancellation of some audio frequencies.
What is desired is a demultiplexer that provides audio frame synchronization in a demultiplexer to align an audio frame representing embedded audio data with video.